Italy is a mini-plate in the grand scheme of worldwide plate tectonics. Current theory is that the heel of the boot was once attached to southern Greece and that there was a minor ocean trapped in the bite between Italy and Europe's present southern coast. Folks tend to think of oceans as huge bodies of water, but the real geological definition of an ocean is simply "water between two continents". The former ocean is irrelevant, however, to this discussion. What's important now is that is that the Italian mini-continent is rotating clockwise. The heel, naturally, broke away from Greece, and the upper part of the Peninsula is riding up onto what now is Switzerland. The most visible results of this "over-riding" collision are the Alps, still being thrust upward by the continuing rotation. South of the Alps there is also upward movement, but less awesome than the constant new crunching of the mountains
So how does "upthrusting" lead to subsidence? In this case, two things are happening simultaneously. The northward movement of the northern Italy has burdened and pushed down the corresponding part of the European plate's southern edge that it is over-riding and made a deep oceanic trench just off the northern part of the Italian eastern coast. Meanwhile that edge of the mini-continent continues to rise as it slides over Europe. So we have a deep trench in the water with relatively high land -- and fast moving streams and rivers -- ashore. Fast rivers erode the land quickly and deposit a lot of silt in the trench. In the northern Adriatic, in fact, the trench is almost full of silt from the Po, Brenta, and Piave river systems. The islands on which Venice sits are the top layers of more than one thousand meters of alluvial. Weather and tide patterns also contribute to the load as the sea periodically washes northward pushing up ridges wherever it can -- for example, just north of the delta of the Po River, where we find the Venice lagoon confined by such barrier island ridges.
In the natural course of events, the eroded silt separates out as it settles. Bigger, heavier gravels and sands tend to settle out closer to land and finer particles move relatively further into the sea. The finest particles are what the storm and tidal surges occasionally washes back over an accumulated layer of gravel and sand. So we get alternating layers of permeable sands and clays -- the clays being compacted fine particles that settle out as the sea recedes. The cycles are close enough that the layers are fairly thin: sand and gravel aquifers that allow fresh water from above the current shoreline alternating with dense clay "aquitards", which prevent or at least retard the fresh water as it tries to escape upward or downward. If nobody bothers such a system, new layers accumulate and, simultaneously, the weight of the new layers compresses those that are below. Depending on how much upstream erosion there is and the density of what is washed out the mouth a river, you can either have a rising or sinking bottom (or, very rarely, one that stays where it is). Without human intervention, the bottom of the Venice would sink, but only a few centimeters per century. New layers almost exactly kept up with compaction, or, said another way, there was only minimal subsidence in the millions of years before Venetians arrived in any appreciable numbers.
That happened in the fifth and sixth centuries, when people from "terraferma" (what Venetians still call the mainland) fled first from the Huns and then the Lombards. Most of the first wave went back ashore after the Huns withdrew in 452 AD, but the second, "Lombard", wave stayed and built on the Islands. It didn't take long for them to realize that the firm building foundations that they knew on terraferma were a thing of the past. Instead of building on bedrock (1000 meters below!) wooden pilings driven into the layers of sand and clay.
Wood of all kinds was used for pilings over the centuries, but, counter-intuitively, softer and less dense woods work best and last longest. This is because their flexibility allows them to be driven down with less chance of cracking and because their more open cell structure allows for quicker and deeper mineral penetration -- the wood is, essentially, quickly petrified. Of the soft woods, pine it best, because its resin deters insect and bacterial damage while the petrifaction process is occurring. Over the centuries, pine tar, other resins, mineral pitches, etc. have been applied to pilings to delay/deter damage.
Venetian builders also easily realized that lighter building materials sink more slowly into the lagoon. Wood was used more extensively than ashore for floors/ceilings and for non-load-bearing walls or partitions. Spans were longer and "overbuilding" (building thicker/stronger than necessary) was very uncommon -- all to save weight. Lighter brick was used instead of stone, at least above the water line where it wouldn't be damaged by sea water, and it was even used below the water line once builders learned to use hydraulic plasters and cements to seal sea water away from subsurface brick.
Despite these efforts, subsidence below structures was always greater that subsidence in areas without structures, and some buildings collapsed and others had to be torn down and built on higher platforms on already embedded pilings. The Campanile in St. Marks Square collapsed in 1902 and was rebuilt within ten years on the same pilings. The simple process of raising buildings on there foundations as their own weight and the weight of newly arrived alluvial sediment bore down lower layers could have gone on indefinitely except for two major events of human intervention.
In 1550, the city fathers thought they could save some money if they didn't have to clean accumulated sediment out of the canals every few years. They knew that the sediments came to the lagoon mostly from the Brenta and Piave Rivers, so they built over the next decades, a system of channels and inland (ashore) canals to divert the flow of these rivers southward into the Po, which would then carry their waters into the Adriatic south if the Lagoon. Some of the northernmost channels, where the Po flowed through its delta, were also blocked. The scheme worked for its purpose, and the Venice (island) canals needed dredging less often. But the measures also broke the equilibrium of the lagoon. Compaction of lower layers continued and new silt didnít keep the lagoon bottom from sinking. It took centuries for the effect to become significant, and, by the time it was noticed, two things prevented reversal of the diversion projects. First, Venice had become impoverished (it's only now recovering) and couldn't afford to pay for returning the rivers to their natural courses. Second, people ashore, especially in the Po Valley, liked the new hydraulic pattern, which gave them more water for their own use. They had a vested interest in keeping the flows on their artificial courses, and Venice had neither the cash nor the political clout to get its rivers back. Fast-forward 400 years for the second event.
Quick industrial development was encouraged on the mainland shore of the lagoon, just west of Venice, in the 1950s and 1960s -- a veritable post-war boom. Population ashore and on the islands also rose dramatically, and the water needs of both the industries and the new people rose even faster -- modern folks use a lot more water per capita than earlier folks. The water was drawn from those sandy, gravelly aquifers between the clay aquitards and the whole area began to sink at twice the rate of previous sinking. There was a wrangle between the water users and gas drillers (working much deeper formations) about who was causing the accelerated subsidence, and neither group was willing to give up their bad habits. The ever-speedy Italian bureaucracy and court systems got involved, and controls were not put in place until the late 1960s.
The interested parties still engage in finger-pointing, but it is a fact that the subsidence returned to its "normal" (that is, post-1550) rate shortly after water extraction was drastically reduced. There are lots of studies that show that the gas drillers were blameless, but those studies mostly seem to come either from the industry or from universities and think-tanks that take grants from the industry.
Both water and gas are now better regulated, but a certain amount of irreversible damage was done: because of the tremendous weight of overlying layers, post-extraction re-pressurization or injection almost never works. Conservationists say that the developers "should have known", and fines may, in fact, some day be levied, etc., but similar problems occurred simultaneously elsewhere in the world in the post-war boom. Nobody really knew enough about such systems in those days to foresee the dangers.
And now we have "global warming"! The Earth's icy bits (glaciers and ice caps) are melting down and sluicing into the seas, and we'll all be under water soon!! Venice and numerous other shoreline and island places will be the first to take the permanent dive!!! Git out the hip boots, Marg'ret, the creek's risin' agin!!!!
There is, of course, still room for argument about whether normal weather and solar-radiation cycles or human activities are the driving force behind the apparent warming. And there is even still room for debate about whether global temperatures are actually rising or whether what looks like a rise might only be an artifact of modern measurement techniques. (Ask me about this stuff when you have an hour or two to hear my presentation.) What can't be denied is that those icy bits are measurably shrinking and that sea levels are rising. Places like Venice need a plan.
And Venice has one: there are only four major gaps (and several smaller ones) in the string of barrier islands that bound the seaward side of the lagoon, and they could be blocked at high water. A system of floating gates would rise automatically at the ever more common "aque alte" periods to prevent surges that erode both buildings and the lagoon bottoms. Venice has had this plan for more than ten years, but the gates have not been installed. Environmentalists, and especially the Italian Green Party, who might be expected to support the plan, are dead set against it, because nobody can accurately predict the impact on the bay ecology. At minimum, they say, gating the lagoon would turn it into a stagnant swamp, and they are probably right. Without the Brenta and Piave River flows, the only thing that occasionally "flushes" the lagoon of its enormous load of human, animal, and, even more poisonous, industrial waste are those same destructive tidal and storm surges.
So there appear to be only two workable solutions: either reverse the errors of the 1550s, or wall it in and pump it out -- make Venice into a miniature Netherlands. Sweep out the canals, pave their bottoms, and get theme park operators to make cars shaped like gondolas, etc. A few water rides could help "ye olde" Venetians and new tourists relive the Venice experience.
The three best general guidebooks for short visits to Venice are:
The Companion Guide to Venice -- available in bookstores in Venice or http://www.amazon.com/exec/obidos/ASIN/1900639246/102-9676007-6590507
The Venice Blue Guide, 6th edition (1998) -- Rome and Venice bookstores or http://www.amazon.com/exec/obidos/ASIN/0393318052/geathotels/102-9676007-6590507 , and
The Touring Club of Italy Heritage Guide to Venice (1999) -- TCI stores in Rome (Via del Babuino 20) or Venice (Piazzale Candiani, 7), other Rome or Venice bookstores, or http://www.amazon.com/exec/obidos/ISBN%3D1885254318/thetouringclubofA/102-9676007-6590507
(There are TCI stores and agencies in all major Italian cities. For the full list, go to http://www.touringclub.it/associarsi/numutili.shtml.)
"VeniceWorld" touring info: http://www.veniceworld.com/, one of dozens of good Venice tourism sites on the Internet (see http://www.google.com/search?client=googlet&q=venice%20italy%20-california%20tourism for a lot more)
There are also hundreds of sites about the many churches of Venice: you can browse though the sites at http://www.google.com/search?client=googlet&q=churches%20Venice or enter the name of a church along with the word "venice" (without the quotation marks) in the search box on the same page.
About.com has a very well respected Venice site: http://goeurope.about.com/mmore.htm
UNESCO's Venice cultural heritage site: http://www.unesco.org/culture/heritage/tangible/venice/html_eng/lagune.shtml
To download the UNESCO Venice Subsidence Case Study from the US Geological Survey: http://wwwrcamnl.wr.usgs.gov/rgws/Unesco/PDF-Chapters/Chapter9-3.pdf. This is the official technical paper.
Some of the incomparable Canaletto views of Venice (early to middle 18th century) are at http://www.kfki.hu/~arthp/html/c/canalett/1/index.html and at http://www.kfki.hu/~arthp/html/c/canalett/2/index.html. Click on the small pix to see a bigger version.
See NASA's "Visible Earth" satellite view: http://earthobservatory.nasa.gov/Newsroom/NewImages/Images/ISS001-ESC-6691.jpg. A one-paragraph caption, technical data on the shot, and a link to a much larger version are at http://visibleearth.nasa.gov/cgi-bin/viewrecord?8331.
Go to http://www.mmdtkw.org/Veneto2001.html for other articles.